Protective film

ABSTRACT

A protective film contains a base material and a pressure-sensitive adhesive layer formed thereon, wherein the surface of the pressure-sensitive adhesive layer has a contact angle with methylene iodide as measured just after contact, θ 1 , of 70° or smaller and a change in contact angle with methylene iodide through 30-second standing, Δθ, of 8% or less.

FIELD OF THE INVENTION

[0001] The present invention relates to a protective film with which themetal layers or metal oxide layers used as electromagnetic shieldingmaterials or the like can be kept in a satisfactory state withoutundergoing fouling or the like.

BACKGROUND OF THE INVENTION

[0002] In plasma display panels (PDPs), CRTs, and the like,electromagnetic shielding is highly important for preventingelectromagnetic noises. In one technique for electromagnetic shielding,a metal such as gold or silver or a metal oxide such as ITO is used asan electromagnetic shielding material for these apparatus to form a thinconductive layer or mesh pattern by vapor deposition or the like. Suchmetal layers or metal oxide layers are generally kept being protectedwith a protective film until use.

[0003] However, when general protective films are used, the surfaces ofthose metal layers or metal oxide layers are apt to be fouled probablybecause these surfaces are especially active. There have been caseswhere this fouling impairs the function of electromagnetic shields.

SUMMARY OF THE INVENTION

[0004] Accordingly, an object of the invention is to provide aprotective film with which the metal layers and metal oxide layers usedas electromagnetic shields or the like can be kept in a satisfactorystate without undergoing fouling or the like.

[0005] The present inventors have found through intensive investigationsthat the fouling which occurs on the surfaces of metal layers or metaloxide layers is considerably influenced by the surfaces of thepressure-sensitive adhesive layers. Specifically, it has been found thatwhen a protective film comprising a base material and formed thereon apressure-sensitive adhesive layer whose surface has a contact angle withmethylene iodide (CH₂I₂) as measured just after contact, θ₁, of 70° orsmaller, preferably from 50 to 70°, and a change in contact angle withmethylene iodide through 30-second standing, Δθ, is 8% or less,preferably 6% or less, is used, then the surface of the metal layer ormetal oxide layer can be prevented from being fouled and can retain asatisfactory state. The invention has thus been achieved.

[0006] The invention provides a protective film comprising a basematerial and a pressure-sensitive adhesive layer formed thereon, whereinthe surface of the pressure-sensitive adhesive layer has a contact anglewith methylene iodide as measured just after contact, θ₁, of 70° orsmaller and a change in contact angle with methylene iodide through30-second standing, Δθ, of 8% or less.

[0007] The change in contact angle through 30-second standing, Δθ, canbe determined using the following equation. Values of contact angle areobtained through measurements with a commercial contact angle meter at23° C. by the θ/2 method.

Δθ=|θ₁-θ₂|×100/θ₁

[0008] Δθ: change in contact angle through 30-second standing (%)

[0009] θ₁: contact angle with methylene iodide as measured just aftercontact (°)

[0010] θ₂: contact angle with methylene iodide as measured at 30 secondsafter contact (°)

[0011] In case where the value of θ₁ exceeds 70° or Δθ exceeds 8%,fouling is apt to occur probably because such a pressure-sensitiveadhesive layer surface shows enhanced bonding to the metal layer ormetal oxide layer surface.

[0012] In this protective film, the main polymer of thepressure-sensitive adhesive layer preferably is a (meth)acrylic polymer.The protective film of the invention is preferably used for the surfaceprotection of an electromagnetic shielding material.

DETAILED DESCRIPTION OF THE INVENTION

[0013] The protective film of the invention comprises a base materialand a pressure-sensitive adhesive layer formed thereon. The protectivefilm may have an interlayer such as, e.g., a primer layer between thebase material and the adhesive layer. The pressure-sensitive adhesivelayer may be made of any material as long as the requirements concerningcontact angle and change in contact angle are satisfied. Namely, thecontact angle between the surface of the pressure-sensitive adhesivelayer in the protective film of the invention and methylene iodide asmeasured just after contact, θ₁, is 70° or smaller, preferably from 50to 70°, and the change in contact angle between the surface andmethylene iodide through 30-second standing, Δθ, is 8% or less,preferably 6% or less.

[0014] The main polymer of the pressure-sensitive adhesive layerpreferably is a (meth)acrylic polymer from the standpoint of obtaining abalance between adhesive properties and suitability for peeling. The(meth)acrylic polymer used here is a polymer obtained by thehomopolymerization or copolymerization of one or more monomers such as,e.g., (meth)acrylic esters having an alkyl group having 1 to 18 carbonatoms optionally together with one or more copolymerizable modifyingmonomers by an appropriate technique such as, e.g., solutionpolymerization or emulsion polymerization.

[0015] Examples of the (meth)acrylic esters and copolymerizablemodifying monomers include (meth)acrylic esters having an alkyl groupsuch as butyl, 2-ethylhexyl, isooctyl, isononyl, ethyl, or methyl andmodifying monomers such as (meth)acrylonitrile, vinyl acetate, styrene,(meth)acrylic acid, maleic anhydride, vinylpyrrolidone, (meth)acrylicesters having a glycidyl group, dimethylaminoethyl group, or hydroxylgroup, (meth)acrylamide, vinylamine, allylamine, and ethyleneimine. Theterm “(meth)acrylic” as used herein refers to “acrylic and/ormethacrylic”; the term “(meth)acrylonitrile” as used herein refers to“acrylonitrile and/or methacrylonitrile”; and the term “(meth)acrylate”as used herein refers to “acrylate and/or methacrylate”.

[0016] Although the (meth)acrylic polymer by itself may be used as thebase polymer of the pressure-sensitive adhesive, a crosslinking agent isusually incorporated into the polymer before use for the purpose ofimproving the cohesive force of the pressure-sensitive adhesive. Acrosslinked structure can be imparted to an acrylic polymer by adding apolyfunctional (meth)acrylate or the like as an internal crosslinkingagent when the acrylic polymer is synthesized or by adding apolyfunctional epoxy or isocyanate compound or the like as an externalcrosslinking agent after the acrylic polymer has been synthesized.Besides these, a crosslinking treatment with exposure to a radiation maybe used. Preferred of these techniques for forming a crosslinkedstructure is the method in which a polyfunctional epoxy compound or apolyfunctional isocyanate compound is incorporated as an externalcrosslinking agent.

[0017] The term “polyfunctional” as used herein means to have afunctionality of 2 or higher. Examples of the polyfunctional epoxycompound include various compounds having two or more epoxy groups inthe molecule. Typical examples thereof include sorbitol tetraglycidylether, trimethylolpropane glycidyl ether,tetraglycidyl-1,3-bisaminomethylcyclohexane,tetraglycidyl-m-xylenediamine, and triglycidyl-p-aminophenol. Examplesof the polyfunctional isocyanate compound include various compoundshaving two or more isocyanate groups in the molecule. Typical examplesthereof include diphenylmethane diisocyanate, tolylene diisocyanate, andhexamethylene diisocyanate.

[0018] Those crosslinking agents can be used alone or in combination oftwo or more thereof. The amount of the crosslinking agents to be usedcan be suitably selected according to the composition and molecularweight of the (meth)acrylic polymer, etc. A crosslinking catalyst ingeneral use for pressure-sensitive adhesives, such as, e.g., dibutyltinlaurate, may be added in order to accelerate the reaction. Furthermore,ordinary additives may be incorporated into the pressure-sensitiveadhesive according to need.

[0019] It is preferred to minimize the incorporation of additives informing a pressure-sensitive adhesive layer. However, suitable additivescan be .incorporated according to need for the purpose of, e.g.,regulating pressure-sensitive adhesive properties. Examples thereofinclude softeners, silicone polymers, acrylic copolymers, tackifiers,antioxidants, hindered-amine light stabilizers, ultraviolet absorbers,and fillers or pigments such as calcium oxide, magnesium oxide, silica,zinc oxide, and titanium oxide. However, such additives preferably areones which cause no damage to the adherend.

[0020] In the invention, it is preferred to take the following measuresin order to satisfy the above-described requirements concerning contactangel and change thereof. For attaining a reduced contact angle, it ispreferred to minimize the amount of units derived from short-side-chainacrylic monomers (e.g., acrylic monomers having a side chain having notmore than 2 carbon atoms) in the (meth)acrylic polymer as long as theproperties of the polymer are not impaired thereby. For attaining areduced change in contact angle, it is preferred to minimize the amountof additives such as low-molecular plasticizers.

[0021] The base material preferably is a thermoplastic film made of asingle polyolefin such as, e.g., polyethylene or polypropylene, amixture of two or more such polyolefins, or another polymer such as anylon, PET, polyimide, polycarbonate, or PTFE. From the standpoints oftransparency, fish-eye diminution, etc., a PET film is preferred.Although the thickness of the base material is generally from 5 to 200μm, preferably from 20 to 100 μm, it is not limited to these values.

[0022] The protective film can be formed by known techniques foradhesive-sheet formation. For example, use may be made of: a method inwhich either a solution of a material for pressure-sensitive adhesivelayer formation in a solvent or a melt of the material is applied to abase material; a method in which a pressure-sensitive adhesive layerformed on a release liner by applying the solution or melt istransferred to a base material; a method in which a material forpressure-sensitive adhesive layer formation is applied to a basematerial by extrusion coating; or a method in which a pressure-sensitiveadhesive layer is laminated to a base material. Although the thicknessof the pressure-sensitive adhesive layer to be formed may be suitablydetermined, it is generally 100 μm or smaller, preferably from 1 to 50μm, especially preferably from 1 to 40 μm. Until the protective film isput to practical use, the pressure-sensitive adhesive is kept beingprotective according to need by provisionally applying a release lineror the like thereto.

[0023] The protective film of the invention is suitable for use in thesurface protection of the metal layer or metal oxide layer of anelectromagnetic shielding material, transparent conductive film, sheetcoated with a vapor-deposited metal, or the like. Especially preferably,it is used for the surface protection of an electromagnetic shieldingmaterial.

[0024] The constitution and effect of the invention will be explainedbelow by reference to Examples thereof, etc.

EXAMPLE 1

[0025] A 20% toluene solution of an acrylic polymer formed from2-ethylhexyl acrylate (70 parts (parts by weight; the same applieshereinafter)), N-acryloylmorpholine (30 parts), and acrylic acid (3parts) was prepared by an ordinary method. To this solution were added 2parts of an epoxy crosslinking agent (Tetrad C, manufactured byMitsubishi Gas Chemical Co., Inc.) and 2 parts of an isocyanatecrosslinking agent (Coronate L, manufactured by Nippon Polyurethane Co.,Ltd.) per 100 parts of the acrylic polymer. The resultant compositionwas applied to a 38-μm PET film in a thickness of 5 μm on a dry basis,and the coating was dried. Thus, a protective film was obtained, whichhad a θ₁ of 56.5° and a Δθ of 1.7%.

EXAMPLE 2

[0026] A 20% toluene solution of an acrylic polymer formed from2-ethylhexyl acrylate (55 parts), vinyl acetate (45 parts), and acrylicacid (3 parts) was prepared by an ordinary method. To this solution wasadded 2 parts of an epoxy crosslinking agent (Tetrad C, manufactured byMitsubishi Gas Chemical Co., Inc.) per 100 parts of the acrylic polymer.The resultant composition was applied to a 38-μm PET film in a thicknessof 5 μm on a dry basis, and the coating was dried. Thus, a protectivefilm was obtained, which had a θ₁ of 58.3° and a Δθ of 1.2%.

EXAMPLE 3

[0027] A 20% toluene solution of an acrylic polymer formed from2-ethylhexyl acrylate (100 parts) and hydroxyethyl acrylate (4 parts)was prepared by an ordinary method. To this solution were added 3.5parts of an isocyanate crosslinking agent (Coronate L, manufactured byNippon Polyurethane Co., Ltd.) and 0.05 part of a reaction catalyst(OL-1, manufactured by Tokyo Fine Chemical Co., Ltd.) per 100 parts ofthe acrylic polymer. The resultant composition was applied to a 38-μmPET film in a thickness of 20 μm on a dry basis, and the coating wasdried. Thus, a protective film was obtained, which had a θ₁ of 62.8° anda Δθ of 3.9%.

EXAMPLE 4

[0028] A 20% toluene solution of an acrylic polymer formed from2-ethylhexyl acrylate (50 parts), butyl methacrylate (50 parts), acrylicacid (0.5 part), and hydroxyethyl acrylate (2 parts) was prepared by anordinary method. To this solution were added 3 parts of an isocyanatecrosslinking agent (Coronate L, manufactured by Nippon Polyurethane Co.,Ltd.) and 0.1 part of a reaction catalyst (OL-1, manufactured by TokyoFine Chemical Co., Ltd.) per 100 parts of the acrylic polymer. Theresultant composition was applied to a 38-μm PET film in a thickness of5 μm on a dry basis, and the coating was dried. Thus, a protective filmwas obtained, which had a θ₁ of 59.1° and a Δθ of 5.1%.

COMPARATIVE EXAMPLE 1

[0029] A 20% toluene solution of an acrylic polymer formed from2-ethylhexyl acrylate (50 parts), methyl acrylate (50 parts), andacrylic acid (2 parts) was prepared by an ordinary method. To thissolution were added 3.5 parts of an isocyanate crosslinking agent(Coronate L, manufactured by Nippon Polyurethane Co., Ltd.), 0.05 partof a reaction catalyst (OL-1, manufactured by Tokyo Fine Chemical Co.,Ltd.), and 15 parts of a plasticizer (dibutoxyethoxyethyl adipate) per100 parts of the acrylic polymer. The resultant composition was appliedto a 38-μm PET film in a thickness of 20 μm on a dry basis, and thecoating was dried. Thus, a protective film was obtained, which had a θ₁of 72.9° and a Δθ of 10.0%.

COMPARATIVE EXAMPLE 2

[0030] The same procedure as in Comparative Example 1 was conducted,except that the plasticizer was omitted. Thus, a protective film wasobtained, which had a θ₁ of 73.5° and a Δθ of 3.8%.

COMPARATIVE EXAMPLE 3

[0031] The same procedure as in Example 3 was conducted, except that 15parts of a plasticizer (dibutoxyethoxyethyl adipate) was added. Thus, aprotective film was obtained, which had a θ₁ of 61.5° and a Δθ of 8.8%.

[0032] The protective films obtained in the Examples and ComparativeExamples were aged at 40° C. for 3 days.

[0033] Evaluation Method

[0034] Each of the protective films obtained in the Examples andComparative Examples was applied to an ITO-coated film by press-bondingthe protective film to the ITO side of the adherend with a laminator ata linear pressure of 78 N/cm. The protective films applied were allowedto stand at 80° C. for 72 hours and then peeled off. Thereafter, theITO-coated films were held to the light from a green lamp to visuallyexamine the films for fouling. The results of this evaluation are shownin the following tables. TABLE 1 Example 1 Example 2 Example 3 Example 4θ₁ (°) 56.5 58.3 62.8 59.1 Δθ (%) 1.7 1.2 3.9 5.1 Fouling not occurrednot occurred not occurred not occurred

[0035] TABLE 2 Comparative Comparative Comparative Example 1 Example 2Example 3 θ₁ (°) 72.9 73.5 61.5 Δθ (%) 10.0  3.8  8.8 Fouling occurredoccurred occurred

[0036] The results given in Tables 1 and 2 show that the protectivefilms of the Examples caused no fouling and were highly satisfactory,whereas the protective films of the Comparative Examples caused fouling.

[0037] While the invention has been described in detail and withreference to specific embodiments thereof, it will be apparent to oneskilled in the art that various changes and modifications can be madetherein without departing from the spirit and scope thereof.

What is claimed is:
 1. A protective film comprising a base material anda pressure-sensitive adhesive layer formed thereon, wherein the surfaceof the pressure-sensitive adhesive layer has a contact angle withmethylene iodide as measured just after contact, θ₁, of 70° or smallerand a change in contact angle with methylene iodide through 30-secondstanding, Δθ, of 8% or less.
 2. The protective film of claim 1, whereinthe pressure-sensitive adhesive layer comprises a (meth)acrylic polymeras a main polymer.
 3. The protective film of claim 1, which is for usein the surface protection of an electromagnetic shielding material. 4.The protective film of claim 2, which is for use in the surfaceprotection of an electromagnetic shielding material.
 5. The protectivefilm of claim 2, wherein the (meth)acrylic polymer is a polymer obtainedby the homopolymerization or copolymerization of one or more(meth)acrylic esters having an alkyl group having 1 to 18 carbon atomsby solution polymerization or emulsion polymerization.
 6. The protectivefilm of claim 2, wherein the (meth)acrylic polymer is a polymer obtainedby the copolymerization of one or more (meth)acrylic esters having analkyl group having 1 to 18 carbon atoms and one or more copolymerizablemodifying monomers by solution polymerization or emulsionpolymerization.
 7. The protective film of claim 5, wherein the(meth)acrylic ester is selected from the group consisting of(meth)acrylic esters having a butyl group, a 2-ethylhexyl group, anisooctyl group, an isononyl group, an ethyl group, or a methyl group. 8.The protective film of claim 6, wherein the (meth)acrylic ester isselected from the group consisting of (meth)acrylic esters having abutyl group, a 2-ethylhexyl group, an isooctyl group, an isononyl group,an ethyl group, or a methyl group.
 9. The protective film of claim 6,wherein the copolymerizable modifying monomer is selected from the groupconsisting of (meth)acrylonitrile, vinyl acetate, styrene, (meth)acrylicacid, maleic anhydride, vinylpyrrolidone, (meth)acrylic esters having aglycidyl group, a dimethylaminoethyl group, or a hydroxyl group,(meth)acrylamide, vinylamine, allylamine, and ethyleneimine.